This invention relates to the polymerization of unsaturated hydrocarbons over Ziegler-type catalyst systems and more particularly to polymerization processes carried out using transition metal catalyst components of such systems having varying internal electron donor-transition metal ratios.
The polymerization of unsaturated hydrocarbons over Ziegler-type catalysts is well known in the art. Such hydrocarbons normally take the form of short chain alpha olefins such as ethylene, propylene and butylene, including substituted alpha olefins such as substituted vinyl compounds, for example, vinyl chloride or vinyl toluene. However, such unsaturated hydrocarbons can also include di-olefins such as 1-3-butadiene or 1-4-hexadiene or acetylenically unsaturated compounds such as methylacetylene or 2-butyne.
Ziegler-type catalysts incorporate a transition metal, usually titanium, zirconium or hafnium, although other transition metals found in Groups 4, 5 and 6 of the Periodic Table of Elements may be employed, which function to provide sites for the insertion of monomer units into growing polymer chains. One type of such polymerization catalysts are the so-called homogeneous catalyst systems in which the transition metal compound is a metallocene comprising one or more substituted or unsubstituted cyclopentadienyl groups coordinated with the transition metal atom forming the situs for polymer growth. Such metallocene-based catalyst systems are the subject of European Patent Application EP 129,368 and U.S. Pat. No. 4,794,096 to Ewen and U.S. Pat. No. 4,892,851 to Ewen et al., the latter two patents disclosing catalysts useful in the polymerization of propylene to form isotactic and syndiotactic polypropylene, respectively.
The more widely used transition metal catalysts are the so-called heterogeneous catalyst systems in which a transition metal halide, usually zirconium, hafnium or titanium, di-, tri-, or tetra-halides, are incorporated with a support structure, principally based upon magnesium or zinc halides, ethoxides or the like. For example, U.S. Pat. No. 4,476,289 to Mayr et al. discloses so called xe2x80x9cactivatedxe2x80x9d titanium tetrahalides, more specifically, titanium tetrachloride, supported on anhydrous magnesium or zinc halides, principally magnesium chloride or magnesium bromide. The transition metal component is used in conjunction with a second component, commonly referred to as a co-catalyst, which as described in the Mayr et al. patent, is a hydride or organometallic compound based primarily upon aluminum, although lithium or magnesium based compounds are also disclosed. A supported catalyst containing yet another component is disclosed in U.S. Pat. No. 4,636,486 to Mayr et al. Here, the titanium compound, which may be a halide, an oxyhalide or an alcoholate in either the di-, tri-, or tetravalent form, is composited with the magnesium support, together with an electron donor compound. Such electron donors, commonly referred to as internal electron donors because they are incorporated as part of the transition metal catalyst component, can be selected from a broad class of compounds including amines, amides, phosphines, ethers, thioethers, alcohol esters, aldehydes, and ketones. As in the case of the aforementioned U.S. Pat. No. 4,476,289 to Mayr, the catalyst system here also includes an organoaluminum co-catalyst such as triethylaluminum, commonly referred to as TEAL. Both of the Mayr et al. patents teach that the molar ratio of the organoaluminum compound and the titanium catalyst component is not critical. In the polymerization of ethylene, such ratio is said to preferably be between 50 and 1,000.
Yet a third component often employed in Ziegler-type catalyst systems is a so-called external electron donor. The external electron donors function similarly as the internal electron donors and in a complimentary or supplementary manner to regulate monomer insertion into the polymer chain growing on the transition metal active sites. Thus, the electron donors can have an impact upon catalyst activity, polymer molecular weight, and polymer morphology as reflected in stereospecificity and physical parameters such as melting point. For example, in the polymerization of propylene, the addition of electron donors under controlled conditions can result in dramatic increases in activity (the amount of polymer produced per unit of catalyst) and in stereoregularity, e.g., an increase in isotactic polymer with a corresponding decrease in atactic. The most widely used external electron donors are organosilicon compounds such as organosilanes and organosiloxanes, including silyl ethers and esters such as alkyl or arylalkyl alkoxysilanes.
The complimentary nature of the internal and external electron donors is addressed in Soga, K. et al., xe2x80x9cEffect of Diesters and Organosilicon Compounds on the Stability and Stereospecificity of Ziegler-Natta Catalystsxe2x80x9d, Transition Metal Catalyzed Polymerizations: Ziegler-Natta and Metathesis Polymerizations, Quirk, R. P., Ed., Cambridge University Press, New York, 1988, pp. 266-279. As discussed in Soga, the concentrations of the internal and external donors in the catalyst system can be adjusted in order to optimize the activity and the stereospecifity of the catalyst. In the experimental work reported there, the transition metal catalyst component comprising titanium tetrachloride supported on magnesium dichloride with an internal donor, e.g., di-N butylphthalate, was slurried in hexane followed by the addition of an external electron donor and triethylaluminum (TEAL) co-catalyst. Soga et al. report on polymerization rates over periods of several hours and isotactic indices measured over periods of several hours for various internal donor, external donor catalyst systems using several kinds of organosilicon compounds at varying concentrations expressed in terms of silicon/titanium mole ratios and TEAL/titanium mole ratios. Corresponding tests in the absence of electron donors were also carried out. Among the various external electron donors used in the Soga et al. experimental work, diphenyldimethoxysilane appeared to have the most efficiency in terms of improving activity and/or stereospecificity of the catalyst system, followed by phenyltriethoxysilane, followed in turn by phenyltrimethoxysilane and then by various tetraalkoxysilanes which were generally less efficient, although still effective. In various tests carried out with and without an external donor, phenyltriethoxysilane, the effect on rate time profiles for the polymerization of propylene varied depending upon the presence and nature of an internal donor. In absence of the external donor, the most active system was one employing ethyl benzoate as the internal donor followed by systems having no internal donor or di-n-butylphthalate or diphthalate grouped fairly closely together with the least active system employing dimethylphenol as the internal donor. Where the external donor was present, di-n-butylphthalate and then ethyl benzoate were the most effective internal donors followed in turn by the supported catalyst which was free of an internal donor and then systems employing diethylphthalate and dimethylphenol as internal donors. The aluminum/titanium mole ratios employed in Soga ranged from about 50 to 200; the silicon/titanium mole ratios range from about 10 to 50. Soga et al. proposed a mechanism to explain the experimental work involving several types of active sites available for production of isotactic polypropylene. The internal donor is hypothesized to coordinate with some of the active sites and to inhibit the formation of specific active sites which are not deactivated by the external donor.
U.S. Pat. No. 4,287,328 to Kikuta et al., is directed to the polymerization of alpha olefins in the presence of multi-component catalyst systems involving a xe2x80x9csolid productxe2x80x9d combined with an organoaluminum compound including, for example, C1-C10 trialkylaluminum, triethylaluminum, alkyl alkyoxyaluminums, and alkylaluminum halides, and an electron donor including various organic acids, alcohols, ethers, aldehydes, ketones, amines, alkenol amines, esters, phosphines, phosphites, thioethers, thioalcohols, silanes, and siloxanes. The xe2x80x9csolid productxe2x80x9d catalyst component is formed by reacting a trivalent metal halide such as aluminum trichloride, aluminum tribromide or ferric trichloride with a divalent metal compound such as magnesium, calcium, or zinc hydroxide or oxide or carbonate with titanium tetrachloride, characterized as an electron acceptor. Numerous orders of additions of the various components are described in Kikuta et al., especially in columns 6 through 9. Conditions of mixing can vary over wide temperature ranges and time intervals, but temperatures are preferably in the range of room temperature to about 100xc2x0 C. The mixing of the various components can be carried out over periods of several minutes to several hours.
U.S. Pat. No. 4,567,155 to Tovrog et al., discloses multi-component catalyst systems useful in the gas phase polymerization of alpha olefins. In Tovrog et al., the catalyst systems comprise two base catalyst components, each containing subcomponents. The first component, identified as component xe2x80x9cAxe2x80x9d comprises a titanium component supported on a hydrocarbon insoluble magnesium component in combination with an electron compound. The second major component is a co-catalyst component, characterized as component xe2x80x9cBxe2x80x9d comprising a trialkylaluminum, an aromatic acid ester and an unhindered secondary amine. Tovrog discloses that the catalyst components may be mechanically activated by comminution prior to use in polymerization. Comminuted catalysts may be pre-polymerized with an alpha olefin before use as a polymerization catalyst component. In the pre-polymerization procedure, comminuted catalysts and an organoaluminum compound co-catalyst are contacted with an alpha olefin under polymerization conditions and preferably in the presence of a modifier such as methyl-p-toluate and an inert hydrocarbon such as hexane, with typical time durations for pre-polymerization and other pretreatment procedures involving periods of minutes up to a few hours.
U.S. Pat. No. 4,767,735 to Ewen et al. discloses a pre-polymerization process carried out over a period of less. than a minute and usually ten seconds or less. In the Ewen et al. procedure, an organic solvent stream such as hexane or heptane is established in a pre-mixing line. To this stream are added sequentially a co-catalyst (TEAL), an external electron donor, diphenyldimethyoxysilane (DPMS) and a supported catalyst component (titanium tetrachloride supported on magnesium dichloride) to form a catalyst system which is then pre-polymerized by contact with propylene for a few seconds. An alternative mode of addition in the Ewen et al. procedure is to add the electron donor to the carrier stream after the addition of the titanium catalyst component, but still before the addition of the propylene. Ewen et al. disclose that the co-catalyst should be present when the electron donor and the transition metal catalyst component contact one another in order to avoid poisoning of the titanium catalyst. In the experimental work described in Ewen et al., one of two titanium chloride catalysts were used in conjunction with TEAL and DPMS under unspecified catalyst concentrations, but at relative amounts of TEAL and DPMS ranging from 2 mmol of TEAL and 0.4 mmol of DPMS (Al/Si ratio of 5) to 2 mmol of TEAL and 0.03 mmol of DPMS (Al/Si ratio of about 67).
High efficiency catalyst systems employing external electron donors which may be characterised generally as sec or tert alkyl or cycloalkyl, alkyl dialkoxy silanes in combination with titanium tetrachloride supported on magnesium based supports derived from dialkoxy magnesium compounds are disclosed in U.S. Pat. No. 4,927,797 to Ewen. By way of example, the supported catalyst may be formulated through the reaction of diethoxy magnesium, titanium tetrachloride, and n-butyl phthalate under appropriate conditions as specified in the patent. A suitable external electron donor here is methylcyclohexyl dimethoxysilane, which is compared with diphenyldimethoxysilane as disclosed in the aforementioned U.S. Pat. No. 4,767,735 to Ewen et al. Examples of internal donors disclosed in U.S. Pat. No. 4,927,797 include amines, amides, esters, ethers, aromatic esters, ketones, nitrites, phosphines, stibines, arsines, phosphoramides, thioethers, thioesters, aldehydes, alcoholates, and salts of organic acids. Specific internal donors used in formulating the catalyst component by contacting a titanium halide with a dialkoxymagnesium support material include dimethylphthalate, diethylphthalate, diisopropylphthalate, dipropylphthalate, dibutylphthalate, diamylphthalate, methylbutylphthalate, and various other phthalate acid esters.
In accordance with the present invention there are provided novel Ziegler-type catalyst systems and processes for the polymerization of olefins with such systems in which the internal electron donor levels are employed to arrive at a desired polymerization characteristic in the polymerization process. The catalyst systems utilized in carrying out the present invention involve transition metal catalyst components incorporating internal electron donors, an external electron donor component and a co-catalyst component which are mixed together to formulate the desired Ziegler-type catalyst systems which are charged to an olefin polymerization reactor. The transition metal catalyst component incorporates an internal electron donor in an amount to provide an internal electron donor/transition metal mole ratio for the catalyst system which corresponds to a desired polymerization characteristic such as yield of the polyolefin product, molecular weight characteristics such as polydispersity or crystallinity such as measured by the xylenes soluble content of a crystalline polymer such as isotactic polypropylene. The catalyst component is mixed with a co-catalyst component such as an alkyl aluminum, specifically, trimethylaluminum or triethylaluminum, the latter being preferred, and an external electron donor component, specifically, an organosilicon such as an organodialkoxysilane, preferably cyclohexymethyldimethoxysilane (CMDS). The catalyst system thus formulated is introduced into a polymerization reactor to effect polymerization of the olefin with the catalyst system. Specific olefins used in the polymerization process are C2-C4 alpha olefins. The preferred olefin is propylene in the production of stereoregular polypropylene. Preferred transition metal catalyst components are halides of titanium, zirconium or hafnium or vanadium, more specifically, a titanium tetrahalide such as titanium tetrachloride supported on a magnesium or zinc-based support.
A specific catalyst system embodying the invention, comprises a transition metal component having an internal electron donor in an amount providing an internal donor/transition metal mole ratio of no more than 2/3, an organoaluminum co-catalyst component in an amount to provide an aluminum/transition metal mole ratio of at least 100, and an organosilicon electron donor component in an amount to provide an aluminum/silicon mole ratio of no more than 200. Preferably the aluminum transition metal mole ratio is at least 200 and the aluminum/silicon mole ratio is 100 or less. In a process embodying the invention, a catalyst system as thus characterized is supplied to a polymerization reactor to effect polymerization of the olefin with the catalyst system. Thereafter, there is provided a second composite catalyst system meeting the aforementioned criteria of internal electron donor/transition metal ratio, aluminum/transition metal mole ratio, and aluminum/silicon mole ratio, but having a different ratio of internal electron donor to transition metal than the first system. The second system is introduced into the polymerization reactor to effect polymerization of the olefin to achieve a different polymerization characteristic than the first catalyst system.